1. Field of the Invention
The present invention relates to a negative-dispersion optical fiber for compensating for chromatic dispersion of a positive-dispersion optical fiber having positive chromatic dispersion in a signal wavelength band, and to an optical transmission line incorporating the same.
2. Related Background Art
An optical transmission system transmits signals of multiple channels through an optical transmission line consisting of optical fibers to enable long-haul and large-capacity communication. Silica based optical fibers commonly applied to the optical transmission lines exhibit their minimum transmission loss near the wavelength of 1.55 xcexcm. On the other hand, an Er-doped optical fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) capable of amplifying signals near the wavelength of 1.55 xcexcm is available for practical use as optical amplification means. For this reason, C-band (1530 nm to 1560 nm) is mainly utilized as a signal wavelength band.
Since EDFA capable of amplifying signals near the wavelength of 1.58 xcexcm was also developed recently, L-band (1570 nm to 1610 nm) is also now being utilized as a signal wavelength band. For realizing much larger capacity transmission, use of S-band (1450 nm to 1530 nm) as a signal wavelength band is also under research.
In addition, wavelength division multiplexing (WDM) optical transmission systems are systems that transmit multiplexed signals of multiple channels included in the foregoing S-band, C-band, or L-band and that enable large-capacity information transmission. Concerning such WDM optical transmission systems, there are needs for further increase of information content and this demands to maintain absolute values of chromatic dispersion small across a wider wavelength band throughout the entire optical transmission line.
However, the optical fibers normally applied to the optical transmission lines have positive chromatic dispersion and the positive sign of dispersion slope in either of the S-band, C-band, and L-band. For example, a standard single-mode optical fiber having the zero dispersion wavelength near the wavelength of 1.3 xcexcm has the chromatic dispersion of about +16 ps/nm/km to +21 ps/nm/km at the wavelength of 1.55 xcexcm. A non-zero dispersion-shifted optical fiber (NZ-DSF) having the zero dispersion wavelength near the wavelength of 1.55 xcexcm has the chromatic dispersion of about +2 ps/nm/km to +12 ps/nm/km at the wavelength of 1.55 xcexcm. These single-mode optical fiber and non-zero dispersion-shifted optical fiber both have positive dispersion slopes in the S-band, C-band, and L-band.
When an optical transmission line is constructed by applying only optical fibers with positive chromatic dispersion (hereinafter referred to as positive-dispersion optical fibers) as described above, the optical transmission line has large cumulative chromatic dispersion. This leads to degradation of waveforms of signals and it thus becomes hard to implement long-haul and large-capacity optical transmission. Therefore, application of optical fibers with negative chromatic dispersion (hereinafter referred to as negative-dispersion optical fibers) is under study in order to compensate for the chromatic dispersion of the positive-dispersion optical fibers (e.g., Japanese Patent Applications Laid-Open No. H6-11620, H8-136758, H8-313750, and so on).
The inventors investigated the above-mentioned prior arts and found the following problem. Namely, it is generally known that the negative-dispersion optical fibers have larger transmission losses than the positive-dispersion optical fibers. Therefore, in the case of using long negative-dispersion optical fibers, there is a problem such that the transmission loss becomes large. According to the knowledge of the inventors, the optical transmission line composed of the positive-dispersion and negative-dispersion optical fibers has such a tendency that average chromatic dispersion on the whole of the optical transmission line is 0 near the zero dispersion wavelength but absolute values of chromatic dispersion increase with deviation from the zero dispersion wavelength. Since the conventional transmission lines had the large deviation of chromatic dispersion in the signal wavelength band as described, there was a limit to implementation of long-haul and large-capacity WDM optical transmission.
The present invention has been accomplished in order to solve the foregoing problem and an object of the invention is to provide a negative-dispersion optical fiber that can compensate in a short length for the chromatic dispersion of the positive-dispersion optical fiber in the signal wavelength band, and an optical transmission line incorporating it and enabling long-haul and large-capacity WDM optical transmission.
In order to accomplish the above object, a negative-dispersion optical fiber according to the present invention has the following properties at the wavelength of 1550 nm: chromatic dispersion D of not more than xe2x88x92150 ps/nm/km and more preferably not more than xe2x88x92180 ps/nm/km; a dispersion slope S satisfying such a condition that a ratio thereof to the chromatic dispersion D, (S/D), is not less than 2.0xc3x9710xe2x88x923/nm nor more than 4.7xc3x9710xe2x88x923/nm; and an effective area of not less than 12 xcexcm2 but less than 25 xcexcm2 and more preferably less than 20 xcexcm2. Another negative-dispersion optical fiber according to the present invention may have the following properties at the wavelength of 1550 nm: chromatic dispersion D of not more than xe2x88x92200 ps/nm/km; and a dispersion slope S satisfying such a condition that a ratio thereof to the chromatic dispersion D, (S/D), is not less than 2.0xc3x9710xe2x88x923/nm nor more than 4.7xc3x9710xe2x88x923/nm.
Since the negative-dispersion optical fiber has the small chromatic dispersion D (the sign of which is negative and the absolute value of which is large) as described above, an optical transmission line composed of a positive-dispersion optical fiber and the negative-dispersion optical fiber can be constructed at a small ratio of the length of the negative-dispersion optical fiber. This suppresses increase of transmission loss due to insertion of the negative-dispersion optical fiber in the optical transmission line and enables construction of the optical transmission line at low cost. Since the foregoing ratio (S/D) is not less than 2.0xc3x9710xe2x88x923/nm nor more than 4.7xc3x9710xe2x88x923/nm, a dispersion slope compensation rate becomes approximately 60% to 140%, whereby it is feasible to make small both respective absolute values of average chromatic dispersion and average dispersion slope on the whole of the optical transmission line and make small the deviation (maximumxe2x88x92minimum) of average chromatic dispersion among wavelengths on the whole of the optical transmission line in the signal wavelength band. The effective area of not less than 12 xcexcm2 is equivalent to or larger than those of the conventional negative-dispersion optical fibers and can effectively restrain the nonlinear optical phenomena. The effective area of less than 25 xcexcm2 and more preferably less than 20 xcexcm2 can effectively restrain increase of loss in the negative-dispersion optical fiber even in a cabled form as a bundle of optical fibers or in a modularized form as wound in coil shape.
The effective area Aeff is given by the following equation, as described in Japanese Patent Application Laid-Open No. H8-248251 (EP 0 724171A2).       A    eff    =      2    ⁢          xe2x80x83        ⁢    π    ⁢          xe2x80x83        ⁢                            (                                    ∫              0              ∞                        ⁢                                          E                2                            ⁢              r              ⁢                              ⅆ                r                                              )                2            /              (                              ∫            0            ∞                    ⁢                                    E              4                        ⁢            r            ⁢                          ⅆ              r                                      )            
In this equation, E represents an electric field caused by propagating light and r a radial distance from the center of the core.
In the negative-dispersion optical fiber according to the present invention, the ratio (S/D) of the dispersion slope S to the chromatic dispersion D is not less than 2.7xc3x9710xe2x88x923/nm nor more than 4.0xc3x9710xe2x88x923/nm. In this case, the dispersion slope compensation rate becomes approximately 80% to 120%, which makes small both the respective absolute values of average chromatic dispersion and average dispersion slope on the whole of the optical transmission line incorporating the negative-dispersion optical fiber and which also makes smaller the deviation of average chromatic dispersion among wavelengths on the whole of the optical transmission line in the signal wavelength band.
In the negative-dispersion optical fiber according to the present invention, the cutoff wavelength at the length of 2 m (CCITT Standard) is preferably not less than 1.0 xcexcm nor more than 2.0 xcexcm. In this case, the bend loss of the negative-dispersion optical fiber can be controlled to a small level.
In the negative-dispersion optical fiber according to the present invention, a transmission loss at the wavelength of 1550 nm is preferably not more than 1.0 dB/km and more preferably not more than 0.7 dB/km. The reason is that increase of the transmission loss on the whole of the optical transmission line can be suppressed more effectively.
For realizing the various properties as described above, the negative-dispersion optical fiber according to the present invention comprises a core region extending along a predetermined axis and having a predetermined maximum refractive index; a first cladding region surrounding the core region and having a refractive index lower than the maximum refractive index of the core region; a second cladding region surrounding the first cladding region and having a refractive index higher than the refractive index of the first cladding region; and a third cladding region surrounding the second cladding region and having a refractive index lower than the refractive index of the second cladding region.
Having this index profile, the negative-dispersion optical fiber is realized with the foregoing various properties and is preferable, particularly, in that the bend loss can be effectively decreased while lengthening the cutoff wavelength. In the negative-dispersion optical fiber, a maximum relative refractive index difference of the core region to the third cladding region is preferably not less than 1.8% nor more than 3.0%. In this case, the bend loss can be decreased readily by lengthening the cutoff wavelength.
An optical transmission line according to the present invention comprises a negative-dispersion optical fiber having the above-mentioned structure and a positive-dispersion optical fiber having the following properties at the wavelength of 1550 nm: chromatic dispersion of not less than +15 ps/nm/km nor more than +21 ps/nm/km; and a dispersion slope of not less than +0.05 ps/nm2/km nor more than +0.07 ps/nm2/km. This optical transmission line is constructed to compensate for the chromatic dispersion of the positive-dispersion optical fiber by the negative-dispersion optical fiber having the chromatic dispersion and dispersion slope both being small (the negative sign and large absolute values) in the signal wavelength band. This structure can decrease the ratio of the length of the negative-dispersion optical fiber in the entire transmission line and can effectively suppress the increase of transmission loss in the entire transmission line eventually. Since the chromatic dispersion and dispersion slope both are compensated in the optical transmission line by the application of the negative-dispersion optical fiber having the various properties as described above, absolute values of chromatic dispersion can be kept small throughout the entire signal wavelength band and it becomes feasible to implement the long-haul and large-capacity WDM optical transmission.
The optical transmission line according to the present invention comprises the negative-dispersion optical fiber (the negative-dispersion optical fiber according to the present invention) and the positive-dispersion optical fiber as described above and is located at least either between a transmitting station and a receiving station, between a transmitting station and a relay station including an optical amplifier or the like, between relay stations, or between a relay station and a receiving station. The negative-dispersion optical fiber incorporated in the optical transmission line may be located in a relay station. Each of the negative-dispersion optical fiber and the positive-dispersion optical fiber constituting the optical transmission line may be constructed of a plurality of optical fibers fusion-spliced to each other.
Further, in the optical transmission line according to the present invention, the deviation of average chromatic dispersion among wavelengths on the whole of the optical transmission line in the wavelength band of 1530 nm to 1560 nm is not more than 0.5 ps/nm/km, the deviation of average chromatic dispersion among wavelengths on the whole of the optical transmission line in the wavelength band of 1450 nm to 1560 nm is not more than 2.0 ps/nm/km, and the deviation of average chromatic dispersion among wavelengths on the whole of the optical transmission line in the wavelength band of 1450 nm to 1610 nm is not more than 4.0 ps/nm/km and more preferably not more than 2.0 ps/nm/km.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.